Cracks
Cracks are the linear ruptures of the metal under stress. Sometimes they appear large and frequently they are narrow separations. The major classification of cracks is:
- Hot cracking
- Cold cracking
- Micro fissuring
- Base metal cracking
- Crater cracking.
Hot cracking: Hot cracking occurs at elevated temperature during cooling and solidifying from the molten stage. The main causes are the restraint on the joint and the presence of higher carbon and sulphur. The other reason is segregation of weld metal due to bead shape. Hot cracks are inter-granular. For avoiding hot cracks, it is essential to keep these joints with least restraint and to have least amount of heat input. Proper matching of electrodes and cooling rate will reduce the probabilities of cracking. The cracks on the craters of weld metal are examples for hot cracking. MPI or LPI are used to detect surface cracks while ultrasonic testing is suitable for internal cracks.
Cold cracking: Cold cracking in steels refers to cracking which occurs even after a few days. In general, cold cracking starts in the Heat Affected Zone (HAZ) of the weld metal. This is primarily associated with combined effects of hydrogen, restraint & martensite formation. Increasing the carbon content in the base metal andthe manganese content in the weld metal also prompts cold cracking. To minimise cracking it is essential to use low hydrogen electrodes and pre heating. Usually MPI for ferrous materials and LPI for non ferrousmaterials are used to detect these cracks at surface.
Micro fissuring: These are very small fissures detectable only at high magnifications. They are caused by hot or cold cracking.
Base metal cracking: Cracking immediately along the weld or brittle fusion zone especially in high carbon or alloy steels areusually hardening cracks. These are the results of too high a cooling rate, leading to martensite formation in the HAZ. They are also called "underbead cracking". Since ductility usually decreases with increasing hardness, base metal cracking is associated with lack of ductility in the HAZ. MPI or LPI are the best suited to detect these types of cracks. Inclusions and laminations present in the base metal are usual reasons for base metal cracking. The factors increasing cracking tendency are
- Increasing wall thickness
- Increasing restraint
- High strength of base or filler metal
- Easy hardenablity of base or filler metal